I have the 31 for some years now, but I know nothing about the Sigma. I'm using it with an old K 10 (bought both of them at the same time, years ago, when Pentax K10D was the new hype ... and the 31 mm significantly cheaper). I'm very satisfied with its overall rendering (sharpness, out of focus rendering) as well as with the build. But keep in mind that its an older design and it's relying on motor for focusing. It's not as fast, nor as accurate in auto-focusing as newer generations. At least this is the case with my copy, on my old K10D. And since it has a very short throw, manual focusing can be a nightmare.

I guess the best advise is to try one ... at least for me, a lens is not only about image quality, but also responsiveness and interaction with camera.